The invention relates to the electrodeposition of zinc, and particularly to a plating bath for plating bright level zinc deposits from aqueous acid plating baths. More particularly, the invention relates to the incorporation into the acid zinc baths of at least one polymeric sulfur-containing compound. The invention also relates to methods for electro-depositing level and bright zinc deposits from such baths.
Considerable attention has been directed over the years to the development of zinc electroplating baths which will produce bright and level deposits of improved quality. Much research has been devoted to improving the overall brightness, the range of allowable current densities, and ductility of the zinc deposits. Until recently, most of the successful zinc plating baths were aqueous alkaline zinc plating baths containing substantial quantities of cyanide which has caused concern regarding toxicity and waste disposal problems.
Accordingly, activity in the plating area has been devoted either to the development of a cyanide-free alkaline plating bath or improvements in acid plating baths. This invention relates to acid plating baths.
Typically, acid plating baths have been based on a suitable inorganic zinc salt such as zinc sulfate or zinc chloride, and the baths usually include buffers such as ammonium sulfate or ammonium chloride, and other additives to promote and improve ductility, brightness, throwing power and covering power. Surface active agents may be included to improve crystal structure, reduce pitting and increase the solubility of other additives.
Aromatic carbonyl-containing compounds generally are incorporated into acid zinc baths as a supplemental brightener additive and for improving fineness of the grain of the zinc deposit. Wetting agents or surfactants have been added to these baths to solubilize or improve the solubility of the carbonyl-containing compounds in the baths, but such wetting agents and surfactants generally result in a bath exhibiting a tendency to foam excessively, particularly on agitation and at the higher current densities often used in zinc plating.